The present invention relates to the art of neutron well logging, and more particularly, to a method and apparatus for verifying the operability of neutron detectors in a neutron logging tool prior to insertion of the tool into a well borehole.
The invention may be advantageously used with neutron tools that employ an armored wireline cable to connect the tool with a surface data processing system during well logging operations. The invention may also be used with tools that perform logging measurements while drilling the well. Such systems are commonly referred to by the acronym "LWD" (Logging While Drilling), where measurement devices, disposed near the bottom of a drilling system, perform measurements while the borehole is being drilled.
Nuclear logging techniques using neutron detectors for making measurements on the subsurface formations surrounding a borehole are well known in the art. See, for example, the article Smith, Calibration, Checking and Physical Corrections For A New Dual-Spaced Neutron Porosity Tool, presented at the 27th Annual SPWLA Symposium, Houston, Tex., June 1986. Neutron logging tools generally include a source of neutrons and at least two neutron detectors, which could be of the so-called proportional counting type containing ionizable gas such as, e.g., helium-3 (He.sup.3). Helium filled detectors used in such tools are available from several sources, such as those manufactured by Reuter-Stokes of Edison Park, Twinsburg, Ohio, as their Models RS-P4-0803-237 and RS-P4-1406-218. In addition to helium filled counters, other types of neutron detectors are also available, such as non-hygroscopic lithium loaded cesium activated glass scintillation counters.
In practice, neutron logging involves the lowering of the neutron tool into the well borehole. The tool irradiates the formations surrounding the tool with high energy (fast) neutrons. The neutron detectors in the tool then detect the irradiated neutrons from the neutron source after they have collided several times with atoms of the formation and are redirected back to the tool. Neutron logs are generated from the detected neutrons. Neutron logs are used primarily for determining the porosity of the formations encountered by the tool. The neutron detectors develop signals proportional to the amount of hydrogen that is found in the formations.
The high-energy neutrons for these type tools are continuously emitted from a radioactive source in the tool. The principal behind the neutron detector technique is to detect those emitted neutrons that are reflected back from the formation material at a much lower energy level, i.e., slower neutrons. The neutrons are slowed by collisions with the formations. Where the mass of the formation materials is essentially equal to the mass of the neutron there is a maximum loss of energy with each collision between the neutron and the formation. In the case of a neutron, the hydrogen nucleus produces the most amount of energy loss since it is closest in mass to that of the neutron. In a random fashion the emitted neutrons as they undergo numerous collisions eventually are rebounded and strike the neutron detectors contained in the tools. Thus, the amount of signals generated by detected neutrons reflected from the formation materials is an indication of the amount of hydrogen that is contained in the formations.
As with any logging tool that is lowered into a well borehole, it is highly desirable that, before committing the tool to the well, all systems should be operational. Well logging is both an expensive and time-consuming operation and, in most cases, requires shutting down the drilling operation in order for the well logging operation to occur. Because a neutron detector has substantially no background counts, it emits no signals in the absence of the neutron source and the presence of materials surrounding the tool. This surrounding material is necessary to produce the energy loss and random diffusion of neutrons back to the detectors for detection. As a consequence, the operator, while the tool is at the surface, has no way of verifying from the signals emanating from the neutron detectors whether or not they are operational. Accordingly, there is a need to be able to verify the operability of the detectors shortly before the tool is lowered into the well borehole. It should be readily apparent that verification of the operability of the neutron detectors also involves verification of the electronics associated with the detectors. It is those circuits that apply voltages to the detector's internal components as well as to process the detector output signals generated in response to the detected neutrons and gamma rays.
Traditionally, detectors have been checked at the well site by placing material containing large quantities of hydrogen near the detectors, thus simulating the environment the detectors will encounter in the well borehole. A neutron source is then placed near the material. The hydrogen slows the neutrons from the source down to a sufficiently for detection by the helium-3 detectors. The counts thus produced by the detectors are monitored in the normal fashion. An example of this technique is described in the Smith article identified above. The primary drawback to this prior art procedure is that it is very time consuming. Another significant disadvantage to this approach is the danger presented by the necessity of providing exterior to the tool the source of neutrons, which is a radioactive material.
An alternate attempt to provide a solution for verification of neutron detector operability may be found in U.S. Pat. No. 5,180,917. That patent suggests depositing a small amount of radioactive material comprising an alpha source, such as uranium or americium radioisotope, inside the helium-filled detectors in the form of a thin foil of metal or other material. The alpha particle emitted by the radioisotope ionizes the gas and produces a signal similar to that produced by a neutron that interacts with the helium-3.
This method has two drawbacks. First, it involves placing a very small quantity of a radioisotope inside the detector, which means that this technique will only work with new detectors. Second, the amount of radioisotope inserted must be kept small so that the neutron count rates obtained during the logging procedure are not significantly affected. Keeping the quantity of radio isotope inside the detector small consequently means that it will take a long time for a significant number of events to occur to permit verification of the detector's operations.
In view of the foregoing, it would be highly desirable to provide the user of both old and new neutron logging tools with a reliable and efficient technique to determine whether the neutron detectors are actually and properly working, especially just prior to committing the tool to the well borehole.